Turbochargers are used in a variety of applications for providing compressed gas for the operation of an internal combustion engine. As an example, with reference to FIG. 1, a turbocharger 101 includes a turbocharger housing and a rotor configured to rotate within the turbocharger housing along an axis of rotor rotation 103 on thrust bearings and journal bearings. The turbocharger housing includes a turbine housing 105, a compressor housing 107, and a bearing housing 109 that connects the turbine housing to the compressor housing. The rotor includes a turbine wheel 111 located substantially within the turbine housing, a compressor wheel 113 located substantially within the compressor housing, and a shaft 115 extending along the axis of rotor rotation, through the bearing housing, to connect the turbine wheel to the compressor wheel.
The turbine housing 105 and turbine wheel 111 form a turbine configured to circumferentially receive a high-pressure exhaust gas stream 121 from an exhaust manifold 123 of an internal combustion engine 125. The turbine wheel (and thus the rotor) is driven in rotation around the axis of rotor rotation 103 by the high-pressure exhaust gas stream, which becomes a lower-pressure exhaust gas stream 127 and is axially released into an exhaust system (not shown).
The compressor housing 107 and compressor wheel 113 form a compressor. The compressor wheel, being driven in rotation by the exhaust-gas driven turbine wheel 111, is configured to compress axially received ambient air 131 into a pressurized air stream 133 that is ejected circumferentially from the compressor. The pressurized air stream is characterized by an increased temperature, over that of the ambient air, due to the compression process, but may be channeled through a convectively cooled charge air cooler 135 configured to dissipate heat from the pressurized air stream, and thereby increase its density. The resulting cooled and pressurized air stream 137 is channeled into an intake manifold 139 on the internal combustion engine.
Numerous modern turbocharger applications have increased pressure ratio and flow range requirements due to the use of exhaust gas recirculation (“EGR”) to reduce emissions, and due to the need for higher engine torque and power ratings. Because conventional single-stage turbochargers have difficulty meeting these requirements, turbochargers are now being connected in series to meet these requirements. An arrangement of in-series turbochargers may add significant size, weight and cost to the overall cost of an internal combustion engine. Additionally, in-series turbochargers must be interconnected by interstage ducting, which can slow overall response time.
Another solution to the need for increased pressure ratio and flow range requirements is the Low Speed Turbocharger (“LST”), which uses two radial compressor wheels on a shaft connected to a single radial turbine. A challenge in designing an LST turbine is that it must provide the required power to efficiently drive both compressors, while having a sufficiently low rotational inertia so as to operate comparably with series turbochargers in terms of transient response.
Accordingly, there has existed a need for a turbocharger apparatus and related methods to provide an increased pressure ratio and an extended flow range, while minimizing cost, size, weight and response time. Preferably these apparatus and related methods provide for accurate control of EGR flow rate over a broad range of engine and compressor operating points, while minimizing engine back-pressure, and thus reducing engine pumping losses. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.